In a bipolar transistor, a junction capacitance exists between the P and N areas of silicon. The junction capacitance arises from the minority charge-storing capacity of a PN junction. For example, an NPN transistor has a base-emitter capacitance at the PN base-emitter junction. Minority (N-type) carries are stored in the P-type base near the junction, and minority (P-type) carriers are stored in the N-type emitter near the junction. The capacitance, sometimes referred to as the diffusion capacitance, represents the ability of the P-type base and the N-type emitter to store minority charge near the junction. The capacitance is determined in part by the bias on the PN junction, and as the forward bias increases, the capacitance also increases.
In some circumstances, the base-emitter capacitance can be significant enough to affect circuit performance. In a circuit using a bipolar transistor as an emitter follower, the base-emitter capacitance can affect the output voltage on the emitter after the base voltage changes. In the emitter follower configuration, when the voltage on the base increases, the voltage on the emitter follows, rising by the same amount. The expected voltage on the emitter is equal to the voltage on the base, minus one base-to-emitter diode voltage drop (V.sub.BE). However under certain circumstances, the voltage on the base may be self-boosted by the effect of the base-emitter capacitance. When the voltage applied to the base rises quickly, and the load connected to the emitter is highly capacitive, a large base-to-emitter voltage can develop. Then as the voltage on the emitter rises, a bootstrap effect increases the base voltage due to the base-emitter capacitance. If the loading is sufficiently capacitive, then the voltage on the base rises beyond the voltage applied. Eventually, the voltage on the emitter follows the voltage on the base above the voltage on the base minus V.sub.BE. If there is no path to discharge the base-emitter capacitor, the voltage on the emitter remains above the desired voltage level.
For some circuit applications, the possibility that the output voltage will self-boost beyond the desired value is harmful to operation of the circuit. For example, in integrated circuit memories, the voltage provided by a write line driver when driving a logic high voltage must be limited to a maximum amount. To allow the output voltage to self-boost would diminish the reliability of the memory.